Ablation system and method of use

ABSTRACT

A system and method for creating lesions and assessing their completeness or transmurality. Assessment of transmurality of a lesion is accomplished by monitoring the impedance of the tissue to be ablated. Rather than attempting to detect a desired drop or a desired increase impedance, completeness of a lesion is detected in response to the measured impedance remaining at a stable level for a desired period of time, referred to as an impedance plateau. The mechanism for determining transmurality of lesions adjacent individual electrodes or pairs may be used to deactivate individual electrodes or electrode pairs, when the lesions in tissue adjacent these individual electrodes or electrode pairs are complete, to create an essentially uniform lesion along the line of electrodes or electrode pairs, regardless of differences in tissue thickness adjacent the individual electrodes or electrode pairs.

RELATED US APPLICATION DATA

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 10/132,379 filed Apr. 24, 2002 and also claimingpriority to Provisional U.S. Patent Application No. 60/287,202, filedApr. 26, 2001 by Francischelli et al., incorporated herein by referencein its entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to tissue ablation devicesgenerally and relates more particularly to devices adapted to ablatelines of tissue, for example for use in conjunction with anelectrosurgical version of the Maze procedure.

[0003] The Maze procedure is a surgical intervention for patients withchronic atrial fibrillation (AF) that is resistant to other medicaltreatments. The operation employs incisions in the right and left atriawhich divide the atria into electrically isolated portions which in turnresults in an orderly passage of the depolarization wave front from thesino-atrial node (SA Node) to the atrial-ventricular node (AV Node)while preventing reentrant wave front propagation. Although successfulin treating AF, the surgical Maze procedure is quite complex and iscurrently performed by a limited number of highly skilled cardiacsurgeons in conjunction with other open-heart procedures. As a result ofthe complexities of the surgical procedure, there has been an increasedlevel of interest in procedures employing electrosurgical devices orother types of ablation devices, e.g. thermal ablation, micro-waveablation, cryo-ablation or the like to ablate tissue along pathwaysapproximating the incisions of the Maze procedure. Electrosurgicalsystems for performing such procedures are described in U.S. Pat. No.5,916,213, issued to Hiassaguerre, et al. U.S. Pat. No. 5,957,961,issued to Maguire, et al. and U.S. Pat. No. 5,690,661, all incorporatedherein by reference in their entireties. Cryo-ablation systems forperforming such procedures are described in U.S. Pat. No. 5,733,280issued to Avitall, also incorporated herein by reference in itsentirety.

[0004] In conjunction with the use of electrosurgical ablation devices,various control mechanisms have been developed to control delivery ofablation energy to achieve the desired result of ablation, i.e. killingof cells at the ablation site while leaving the basic structure of theorgan to be ablated intact. Such control systems include measurement oftemperature and impedance at or adjacent to the ablation site, as aredisclosed in U.S. Pat. No. 5,540,681, issued to Struhl, et al.,incorporated herein by reference in its entirety.

[0005] Additionally, there has been substantial work done towardassuring that the ablation procedure is complete, i.e. that the ablationextends through the thickness of the tissue to be ablated, beforeterminating application of ablation energy. This desired result is sometimes referred to as a “transmural” ablation. For example, detection ofa desired drop in electrical impedance at the electrode site as anindicator of transmurality is disclosed in U.S. Pat. No. 5,562,721issued to Marchlinski et al, incorporated herein by reference in itsentirety. Alternatively, detection of an impedance rise or an impedancerise following an impedance fall are disclosed in U.S. Pat. No.5,558,671 issued to Yates and U.S. Pat. No. 5,540,684 issued to Hassler,respectively, also incorporated herein by reference in their entireties.

[0006] Three basic approaches have been employed to create elongatedlesions using electrosurgical devices. The first approach is simply tocreate a series of short lesions using a contact electrode, moving italong the surface of the organ wall to be ablated to create a linearlesion. This can be accomplished either by making a series of lesions,moving the electrode between lesions or by dragging the electrode alongthe surface of the organ to be ablated and continuously applyingablation energy, as described in U.S. Pat. No. 5,897,533 issued toMulier, et al., incorporated herein by reference in its entirety. Thesecond basic approach to creation of elongated lesions is simply toemploy an elongated electrode, and to place the elongated electrodealong the desired line of lesion along the tissue. This approach isdescribed in U.S. Pat. No. 5,916,213, cited above and. The third basicapproach to creation of elongated lesions is to provide a series ofelectrodes and arrange the series of electrodes along the desired lineof lesion. The electrodes may be activated individually or in sequence,as disclosed in U.S. Pat. No. 5,957,961, also cited above. In the caseof multi-electrode devices, individual feedback regulation of ablatedenergy applied via the electrodes may also be employed. The presentinvention is believed useful in conjunction with all three approaches

SUMMARY OF THE INVENTION

[0007] The present invention is directed toward an improved system forcreating lesions and assessing their completeness or transmurality. Inparticular, the preferred embodiments of the invention are directedtoward an improved system for creating elongated lines of lesion andassessing their completeness or transmurality. In the preferredembodiment as disclosed, the apparatus for producing the lesions is anelectrosurgical device, in particular a saline-irrigated bipolarelectrosurgical forceps. However, the mechanism for assessing lesiontransmurality provided by the present invention is believed useful inother contexts, including unipolar radio-frequency (RF) ablation and RFablation using catheters or hand-held probes. The mechanism forassessing transmurality may also be of value in the context of othertypes of ablation systems, particularly those which ablation occurs inconjunction with an induced rise in tissue temperature such as thoseapplying ablation energy in the form of microwave radiation, light(laser ablation) or heat (thermal ablation).

[0008] According to the present invention, assessment of transmuralityof a lesion is accomplished by monitoring the impedance of the tissue tobe ablated. The inventors have determined that, particularly in the caseof a saline-irrigated electrosurgical ablation device, tissue impedancefirst falls and then reaches a stable plateau, during which portion thelesion is completed. Thereafter, the impedance rises. Rather thanattempting to detect a desired drop or a desired increase impedance asdescribed in the above cited Yates, Hassler and Marchlinski patents, thepresent invention detects completeness of a lesion in response to themeasured impedance remaining at a stable level for a desired period oftime, hereafter referred to as an impedance plateau. In the context ofRF ablation, measurement of impedance may be done using the ablationelectrodes or may be done using dedicated electrodes adjacent to theablation electrodes. In the context of the other types of ablationdiscussed above, impedance measurement would typically be accomplishedby means of a dedicated set of impedance measurement electrodes.

[0009] In the context of RF ablation, the invention is believed mostvaluable in the conjunction with an ablation device having multiple,individually activatable electrodes or electrode pairs to be arrangedalong a desired line of lesion. In this context, the mechanism fordetermining transmurality of lesions adjacent individual electrodes orpairs may be used to deactivate individual electrodes or electrodepairs, when the lesions in tissue adjacent these individual electrodesor electrode pairs are complete. This allows the creation of anessentially uniform lesion along the line of electrodes or electrodepairs, regardless of differences in tissue thickness adjacent theindividual electrodes or electrode pairs. However, the invention is alsobelieved useful in conjunction with assessment of transmurality oflesions produced by devices having only a single electrode or singleelectrode pair. Similar considerations apply to the use of the presentinvention in the contexts of other types of ablation as listed above.

[0010] In yet another aspect of the invention, RF power delivery canalso be controlled in order to assess lesion transmurality. The ablationwill commence at a first power level and will be increased to a secondpower level as certain conditions are satisfied. The power level canalso be increased from the second power level to a third power level ascertain conditions are satisfied. The increase in power level cancontinue in the same stepwise manner, increasing the power level from anN power level to an n+1 power level until a set of conditions are metthat indicates that transmurality has been achieved and/or that thepower should be turned off. Conditions for an increase in power levelcan include one or more of: the detection of a plateau in impedance orthe achievement of a maximum permitted time for that power level.Conditions indicating that transmurality has been achieved can includeone of: the lack of change in detected impedance in response to thechange in power level or the detection of a rapid rise in impedance. Iftransmurality is detected by satisfying one of these conditions, thepower is turned off and the ablation is complete. If a conditionindicating that transmurality has been achieved is not detected, such asa drop in detected impedance in response to the change to the higherpower level, ablation is continued at the higher power level until theconditions are met for again increasing the power level or conditionsindicating transmurality are met. This process may be continued in astepwise fashion until the conditions for detection of transmurality aredetected, a predetermined number of plateaus in impedance have beendetected or until an overall maximum time for the ablation is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a plan view of a type of electrosurgical hemostat thatmay be used in conjunction with the present invention.

[0012]FIGS. 2a and 2 b illustrate alternative electrode configurationsfor a hemostat generally according to FIG. 1.

[0013]FIG. 3 illustrates the fall and plateau of impedance measuredacross tissue ablated using a bi-polar, saline irrigated electrosurgical hemostat.

[0014]FIG. 4 is a functional block diagram of an RF generatorappropriate for use in practicing the present invention, particularlyadapted for use in conjunction with an ablation system employingmultiple, individually activatable electrodes or multiple electrodepairs.

[0015]FIG. 5a is a functional flow chart illustrating a first mode ofoperation of the device illustrated in FIG. 4 in practicing the presentinvention.

[0016]FIG. 5b illustrates an alternative mode of operation of a deviceas in FIG. 4 in practicing the present invention.

[0017]FIG. 6 illustrates an additional alternative mode of operation ofa device as in FIG. 4 in practicing the present invention.

[0018]FIG. 7 illustrates a first mode of operation of a device as inFIG. 4 to activate and deactivate of individual electrodes or electrodepairs.

[0019]FIG. 8 illustrates a second mode of operation of a device as inFIG. 4 to activate and deactivate individual electrodes or electrodepairs.

[0020] FIGS. 9-12 are graphs illustrating modes of operation for adevice according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021]FIG. 1 is a plan view of a bipolar, saline irrigatedelectrosurgical hemostat of a type that may be employed in conjunctionwith the present invention. The hemostat is provided with elongatedhandles 11 and 12 and a lock mechanism 14, similar to a conventionalsurgical hemostat. The handles are connected to one another by pivot orhinge 16, and continue distally in the form of elongated jaws 18 and 19.Jaws 18 and 19 carry an elongated electrode or series of electrodes 24,25, respectively, to which ablation energy, e.g. RF energy is applied bymeans of conductors 21 and 22. The electrodes are adapted to beirrigated by a saline solution or other conductive fluid along theirlength, provided via inlet tubes 20 and 23. In operation, tissue to beablated is compressed between the jaws, and RF energy is applied betweenthe electrode or electrode sets 24 and 25, as generally described inU.S. Pat. No. 6,096,037 issued to Mulier et al incorporated herein byreference in its entirety.

[0022]FIG. 2a shows a first embodiment of an electrode arrangement for ahemostat generally as illustrated in FIG. 1. Illustrated componentscorrespond to identically numbered components in FIG. 1. In thisembodiment, electrodes 24 and 25 take the form of elongated coilelectrodes 30 and 32, mounted around porous tubes 34 and 36, throughwhich saline or other conductive fluid is delivered. The arrangement ofthe electrodes may also be reversed, for example placing coils 30 and 32within elongated porous tubes 34 and 36, to accomplish a similar result.Alternatively, any other arrangement for providing an elongatedelectrode and delivery of saline solution along the length thereof maybe substituted. The particular configuration of the electrode is notcritical to the present invention. For example, irrigated electrodescorresponding to those described in U.S. Pat. No. 6,096,037 issued toMulier, et al., U.S. Pat. No. 5,876,398 issued to Mulier, et al., U.S.Pat. No. 6,017,378 issued to Brucker, et al or U.S. Pat. No. 5,913,856issued to Chia, et al., all incorporated herein by reference in theirentireties may also be substituted. It should also be noted that whilethe electrode system as illustrated in FIG. 2a is a bipolar system, theinvention may also be employed in conjunction with unipolar electrodesand/or in the form of a probe or a catheter. In some embodiments,irrigation of the electrodes may be omitted.

[0023]FIG. 2b illustrates an alternative embodiment of an electrodesystem for a hemostat generally as illustrated in FIG. 1. In this case,rather than a single pair of electrodes, multiple electrode pairs areprovided. The electrode pairs comprise coil electrodes 40 and 42, 44 and46, 48 and 50, 52 and 54, and 56 and 58. However, other pairings ofelectrodes might also be substituted, for example, pairing electrodes 40and 44, electrodes 48 and 52 or the like. In this embodiment, theelectrode pairs are mounted around porous plastic tubes 60 and 62through which saline or other electrically conductive fluid isdelivered. As in the case with the embodiment of FIG. 2a, thearrangement of these electrodes may readily be reversed, placing theelectrodes within the lumen of plastic tube 60 or 62 and any otherarrangement providing multiple, irrigated electrodes may also besubstituted. As in the case of the embodiment of FIG. 2a, unipolarelectrodes might be substituted for the multiple bipolar pairs asillustrated and/or the invention may be practiced in conjunction with amulti-electrode probe or catheter. While the particular arrangement ofelectrodes is not believed critical to practicing the present invention,it is believed that the invention may be most beneficially practiced inconjunction with a set of linearly arranged bipolar electrode pairs asillustrated in FIG. 2b.

[0024] In use, the hemostat is arranged so that the tissue to be ablatedis located between the jaws 18 and 19, and pressure is applied in orderto compress the tissue slightly between the jaws to ensure goodelectrical contact. All electrode pairs may be activated individuallyand may be individually deactivated when the lesions between theindividual electrode pairs are completely transmural. Alternatively,electrode pairs could be activated sequentially, with one pairdeactivated upon a detection of a complete lesion between the electrodepair, followed by activation of the next sequential electrode pair.Corresponding use of the invention in conjunction with a series ofunipolar electrodes, for example corresponding to electrodes along oneof the two jaws in conjunction with a remote ground plate or a similarseries of individually activatable electrodes on a catheter or probe inconjunction with a ground plate is also possible.

[0025]FIG. 3 is a graph illustrating measured impedance versus timeacross tissue located between the electrodes of an irrigated bipolarhemostat as illustrated in FIG. 1. FIG. 3 illustrates the drop inimpedance followed by an impedance plateau, ultimately followed by animpedance rise. The impedance plateau is the primary indicator oftransmurality employed by the present invention. Following the impedanceplateau, as tissue is desiccated or as steam or microbubbles forms inthe tissue, an impedance rise will generally occur. In some embodimentsof the invention, the detection of this rise in impedance is employed asan alternative or mechanism for assessing transmurality and/or as asafety mechanism in order to assure shut off of the ablation electrodesbefore excessive physical damage to the tissue results.

[0026]FIG. 4 is a functional block diagram illustrating one embodimentof an RF generator system for use in conjunction with the presentinvention. In this embodiment, separately controllable RF outputs areprovided for individual ablation electrodes or electrode pairs on anassociated RF ablation device, for example as in FIG. 2B. The RFgenerator could of course also be used with ablation devices having onlya single electrode or electrode pair as in FIG. 2A. With the exceptionof the electrogram amplitude measurement circuits discussed below, thegenerator corresponds generally to that described in conjunction withFIG. 14 of the '961 patent issued to Maguire, et al., cited above. TheRF generator disclosed in the '961 patent provides feedback control ofRF power based upon either measured power (constant power) ortemperature. The present invention is somewhat easier to implement inconjunction with the constant power mode, but may also be adapted to atemperature-regulated mode or to other feedback power regulationmechanism.

[0027] Display 804 and controls 802 are connected to a digitalmicroprocessor 800, which permits interface between the user and theremainder of the electrical components of the system. Microprocessor 800operates under control of stored programming defining its operationincluding programming controlling its operation according to the presentinvention, as discussed in more detail below. Microprocessor 800provides control outputs to and receives input signals from theremaining circuitry via address/data bus 806. In particular, themicroprocessor 800 provides for monitoring of power, current, voltage,impedance and temperature. As necessary, the microprocessor will providethis information to the display 804. Additionally, the microprocessor800 permits the user to select the control mode (either temperature orpower) and to input the power set point, temperature set point, and atimer set point to the system. The primary source of power for theradio-frequency generator may be a 12 V battery rated at 7.2ampere-hours. Alternatively, the device may be AC powered. A back-upbattery (not shown) such as a lithium cell may also be provided toprovide sufficient power to the microprocessor 260 to maintain desiredmemory functions when the main power is shut off.

[0028] The power supply system as illustrated includes a desired number“M” of individually controllable RF power supplies and receivestemperature inputs from a desired number “N” of temperature sensingdevices in the ablation device, illustrated schematically at 838 andreceives temperature inputs from a desired number “M” of impedancemonitoring circuits. Each RF power supply includes a transformer (822,824, 826), a power control circuit (810, 812, 814) and a powermeasurement circuit (816, 818, 820). A crystal-locked radio-frequencyoscillator 264 generates the switching pulses, which drive both thepower transformers (822, 824, 826) and the power controllers (810, 812,814). Power controllers (810, 812, 814) may be analog controllers whichoperate by pulse-width modulation by comparing a power set point signalfrom microprocessor 800 with an actual power signal generated by a powermeasurement circuit (816, 818, 820), which may, for example, include atorroidal transformer coupled to the power output from the associatedtransformer (822, 824, 826). The power measurement circuits (816, 818,820) multiply the output current and voltage and provide the resultingactual power signal to both the power controllers (810, 812, 814) andthe microprocessor 800.

[0029] The RF power output of the transformers (822, 824, 826) isprovided to inter face board 808, and thereby is provided to theablation electrode or electrodes on the ablation device 838. Separateanalog comparator circuits (not illustrated) may also be provided formonitoring the output of the power measurement circuits (816, 818, 820),in order to shut-off current to the output transformers (822, 824, 826)if the power exceeds a limit, typically 55 watts. Power transformers(822, 824, 826) may include center taps, which receive the outputs ofthe power controllers (810, 812, 814). Secondary windings of thetransformers (822, 824, 826) may provide for continuous monitoring ofthe applied voltage in order to permit the power calculations by powermeasurement circuits (816, 818, 820).

[0030] The illustrated power RF generator system employs softwarecontrolled temperature processing, accomplished by micro processor 800,which receives the “N” temperature input signals from temperaturemeasurement circuits (828, 830, 832), each of which are coupled to acorresponding temperature sensor in ablation device 838 by means of anelectrical connector, illustrated schematically at 836 and interfacecircuit 834. If programmed to operate in the temperature controlledmode, processor 800 receives the “N” temperature signals and, based uponthe indicated temperatures, defines power set points for each of thepower control circuits (810, 812, 814), which in the manner describedabove control the power levels applied to electrodes on the catheterthrough interface 834. Processor 800 may also selectively enable ordisable any of the “M” provided RF power sup plies, in response toexternal control signals from controls 802 or in response to detectedanomalous temperature conditions.

[0031] In addition to the circuitry as described above and disclosed inand disclosed in the Maguire, et al. '961 patent, the apparatus of FIG.4 includes multiple impedance monitoring circuits ZM1, ZM2. . . ZMM(842, 844 and 846 respectively), which may operate as described in U.S.Pat. No. 5,733,281, issued to Nardella, or U.S. Pat. No. 5,863,990,issued to Li, also both incorporated herein by reference in theirentireties to measure an impedance between electrodes on a RF ablationdevice. Impedance may be measured between the ablation electrodes orbetween electrodes located adjacent the ablation electrodes, asdescribed in U.S. Pat. No. 5,558,671, incorporated by reference above.Individual impedance measurements made by measurement circuits 842, 844and 846 are provided to the address/data bus 806 and thence tomicroprocessor 800 for analysis to determine whether the impedance overtime, indicates that the lesion associated with the measured impedanceis completely transmural. As discussed in more detail below, adetermination of transmurality is made in response to a detection of aseries of impedance measurements that are relatively constant, over adesired period of time or over a defined number of successive impedancemeasurements. In some embodiments, an abrupt rise in impedance may alsobe employed to terminate delivery of ablation energy.

[0032] As an alternative to dedicated impedance monitoring circuits, themicroprocessor may employ voltage and current measurements of impedancemeasurement signals generated by the power transformers (822, 824, 826)to derive impedance and may use such derived impedance values inconjunction with the present invention. For example, measurement ofimpedance in this fashion is disclosed in U.S. Pat. No. 5,540,681,issued to Struhl, et al, cited above or U.S. Pat. No. 5, 573,533, issuedto Struhl, also incorporated herein by reference in its entirety.

[0033] In cases in which an alternative ablation energy generationapparatus is employed, particularly those in which a rise in tissuetemperature is induced, e.g. laser, microwave or thermal ablation, theRF generation circuitry of FIG. 4 would be replaced with a correspondingalternative ablation energy generation apparatus. The measurement oftissue impedance and its use according to the present invention,however, may still be useful in conjunction with these alternativeablation energy generation systems.

[0034]FIG. 5A is a functional flow chart illustrating the operation of adevice as in FIG. 4, according to the present invention. The flow chartof FIG. 5A illustrates operation of the device of FIG. 4 to controlprovision of RF energy to an individual electrode or electrode pair. Inthe event that multiple electrodes or electrode pairs are employed, thecontrol methodology of FIG. 5A would be applied to each electrode orelectrode pair individually, as discussed in more detail below inconjunction with FIGS. 7 and 8.

[0035] The flow chart of FIG. 5A illustrates a method of assessingtransmurality and terminating delivery of ablation energy to anelectrode or an electrode pair responsive to detection of a plateau inimpedance in conjunction with a detected drop in impedance. Followingthe detection of a plateau in conjunction with the required impedancedrop, the device waits a defined time period and then terminates theapplication of ablation energy to the associated electrode pair.Measurement of impedance in tissue associated with the electrode pairmay be made using the ablation electrodes themselves or electrodeslocated in proximity to the ablation electrodes, for example asdescribed in the Yates '671 patent, incorporated by reference above.

[0036] After initialization at 200, the microprocessor 800 (FIG. 4)initiates delivery of ablation energy at 201 and causes the impedancemeasurement circuitry associated with the electrode or electrode pairbeing evaluated or derives impedance based on applied voltage andcurrent as discussed above to acquire a base line or initial impedanceZ_(i), which may be, for example the maximum impedance during the firstthree seconds of ablation. At 202 and 204 counts “n” “m” are reset tozero. The microprocessor thereafter acquires a current impedance valueZ_(n) at 206. The value of “n” is incremented at 208 and compared with apredefined value “a” at 210 to determine whether a sufficient number ofimpedances have been measured in order to calculate the rate of changeof impedance (dZ/dt). For example, dZ/dt may be calculated by taking themeasured impedance Z_(n) and comparing it to the preceding measuredimpedance Z_(n−1), in which case n would have to be greater or equal to2 in order for dZ/dt to be calculated. Alternatively, dZ/dt may becalculated by taking the measured impedance Z_(n) and comparing it tothe previously measured impedance Z_(n−2), in which case n would have tobe greater or equal to 3 in order for dZ/dt to be calculated. If“n” isless than “a” at 210, the microprocessor waits an impedance samplinginterval Δt₁ which may be, for example, 0.2 seconds, and then triggersan impedance measurement again at 206. This process continues untilthere are a sufficient number of collected impedance measurements tocalculate dZ/dt at 212.

[0037] At 212, the microprocessor 800 employs the stored impedancemeasurements to calculate dZ/dt, which may, for example, be calculatedas equal to (1/(2Δt₁))(Z_(n)−Z_(n−2)) The absolute value of dZ/dt, i.e.,|dZ/dt|_(n) is employed to assess whether or not an impedance plateauhas been reached at 214. The microprocessor checks at 214 to determinewhether |dZ/dt|_(n) is less than a defined constant b, indicative of aminimal rate of change of impedance. In the case of an elongated, fluidirrigated ablation electrode similar to that illustrated in FIG. 2A, forexample using an electrode of approximately 5 centimeters in lengthoperated in a constant power mode with a power of less than 27 watts, anappropriate value of “b” might be 1.3. For other electrodeconfigurations and other power levels, the value of “b” may have to beadjusted. Other power control modes, e.g. temperature controlled wouldsimilarly require adjustment of this parameter. The value of “b” andother parameters employed to determine transmurality using the presentinvention can be determined empirically in the laboratory by applyingthe specific electrode set and RF generation system in question to testspecimens, reading impedances at defined sample intervals and using theresults to optimize the various parameters.

[0038] In the event that |dZ/dt|_(n) is sufficiently small in value at214, the count “m” is incremented at 216 and m is compared to a thirdconstant “c” which sets forth the defined number of low value|dZ/dt|_(n) measurements required to detect an impedance plateau. Forexample, in a system as described herein, “c” may be 6-12.Alternatively, rather than requiring an entire series of measured|dZ/dt|_(n) values to be less than “b”, a requirement that a definedproportion of the |dZ/dt|_(n) values must be less than “b” may besubstituted, e.g. 8 of 12, or the like.

[0039] If the number of low values of |dZ/dt|_(n) is less than “c” at218, the microprocessor waits the impedance sampling interval Δt₁ at 220and continues to acquire successive impedance measurements untilsufficient number of sequential low values of |dZ/dt|_(n) have occurredat 218. At that point, the microprocessor then compares the currentimpedance value Z_(n) with the initial impedance value Z_(i) todetermine whether a sufficient impedance drop has occurred. If not, themicroprocessor waits for the next impedance sampling interval at 220 andcontinues to collect impedance measurements and make calculations of|dZ/dt|_(n) until such time as an impedance plateau is recognized at 218and a sufficient impedance drop is recognized at 220. When both of thesecriteria have been met at 220, the microprocessor than waits for anadditional time interval Δ_(t2) to assure completeness of the lesion at222 and thereafter terminates the provision of ablation energy to thespecific electrode pair being regulated at 224 and the ablation processwith regard to that electrode or electrode pair is terminated at 226.

[0040]FIG. 5B illustrates an additional set of operations forimplementing a transmurality measurement method generally as in FIG. 5A.The operations of FIG. 5B may either be a substituted for step 222 ofFIG. 5A or alternatively may be performed before or after step 222. Inthe additional operations illustrated in FIG. 5B, the microprocessorincreases the power to the electrodes under consideration slightly at228 and sets a timer at 230. The microprocessor then causes theimpedance measurement apparatus associated with the electrodes underconsideration to measure current impedance at 234 and calculate|dZ/dt|_(n) at 236, in the same fashion as discussed above. In the eventthat the value of |dZ/dt|_(n) is greater then a defined constant “e” at240, the microprocessor returns to 204 and resets the value of “m” tozero, essentially reinitializing the search for an impedance plateau.The value of “e” may be equal to “b” or may be different. If the valueof |dZ/dt|_(n) is sufficiently small at 240, the microprocessor checksat 242 to determine whether the timer has expired. If not, themicroprocessor waits the impedance sampling interval Δ_(t1) at 238 andcontinues to collect impedance values and calculate |dZ/dt|_(n) valuesuntil expiration of the timer at 242, at which point it eitherterminates ablation at 224 or initiates the waiting period Δ_(t1) at222.

[0041]FIG. 6 illustrates a second basic approach to assessment oftransmurality and control of delivery of ablation energy to an electrodepair. In the same fashion as for FIGS. 5a and 5 b, the procedure definedby the flow chart of FIG. 6 it should be understood to be employed inconjunction with an impedance measurement circuit with a singleelectrode pair, which procedure would be repeated for other electrodesor electrode pairs, if present. After initiation at 300, the value of“n” is set to Zero at 302 and a timer is initiated at 304, used todetermine that a sufficient amount of time has passed prior totermination of ablation. For example, in the context of a bipolarirrigated hemostat similar to FIGS. 1 and 2a as described above, havingelectrode lengths of 2 to 5 centimeters and receiving RF energy at alevel of 27 watts or less, ten seconds may be an appropriate timeinterval for the timer initiated at 304.

[0042] The microprocessor than measures the current impedance Z_(n), at310 increments the value of“n” at 310 and checks at 314 to determinewhether an adequate number of impedance measurements have beenaccumulated to make a calculation of dZ/dt at 314, in the same fashionas discussed above in conjunction with FIGS. 5a and 5 b. If aninadequate number of samples have been collected, the microprocessorwaits the impedance sampling interval Δ_(t) at 308 and continues tocollect impedance measurements until an adequate number “a” ofmeasurement have been collected. In the specific example presented, “a”may be set equal to 5 and Δ_(t) may be 0.2 seconds. After an adequatenumber of impedance measurements have been collected at 314, themicroprocessor calculates the value of dZ/dt_(n) and |dZ/dt|_(n) at 316.In conjunction with the specific mechanism of plateau detectionillustrated in FIG. 6, filtered or average impedance values Z_(a) may beemployed to calculate dZ/dt and |dZ/dt|_(n).

[0043] For example, at 316, the microprocessor may calculate the valueof dZ/dt according to the following method. The microprocessor mayemploy a 5 point moving average filter to produce an average impedancevalue Z_(a), which is equal to(Z_(n)+Z_(n−1)+Z_(n−2)+Z_(n−3)+Z_(n−4))/5. The value of dZ/dt_(n) and|dZ/dt|_(n) may be calculated in the same fashion as in conjunction withthe flow charts of FIGS. 5A and 5B discussed above, substituting theaveraged impedance values Z_(a) for the individual impedance valuesZ_(n) as employed in the previous flow charts. In this case, dZ/dt_(n)would be (1/(2Δ_(t))) (Z_(n)−Z_(n−2)) and |dZ/dt|_(n) would be theabsolute value of dZ/dt.

[0044] At 318, microprocessor 308 may attempt to determine whether animpedance plateau has been reached employing the following method. Themicroprocessor reviews the preceding 15 measurements of dZ/dt_(n), and|dZ/dt|_(n) and applies three criteria to those values, all three ofwhich must be met in order for an impedance plateau to be detected. Fora fluid irrigated hemostat as described, the rules may be as follows:for n=1 to 15; |dZ/dt|_(n) must be less than or equal to 1 for all 15;and for n=1 to 15 the value of dZ/dt_(n) must be less than or equal to0.75, for thirteen of the 15; and for n=1 to 15, |dZ/dt|_(n) must beless than or equal to 0.5, for 10 of the 15. If all of these criteriaare met at 318, the microprocessor checks at 319 to determine whether anadequate period of time has elapsed since ablation was initiated, ifnot, the microprocessor waits for the impedance sampling interval Δ_(t)at 312 and continues to measure impedances and calculate values of dZ/dtaccording to the above described method until both a plateau is presentand the defined time period “b” has elapsed at 319. After the requiredtime period at “b” at 319 has elapsed, delivery of ablation energy tothe associated electrode or electrode pair is terminated at 322 and theablation process ceases at 324 as to that electrode pair.

[0045] In this embodiment, in the event that an impedance plateau failsto manifest itself, the microprocessor may nonetheless safely terminateprovision of ablation energy to the electrode pair under considerationin response to a detected rapid rise in impedance, which normallyfollows the impedance plateau. In the event that a plateau is notdetected at 318, the microprocessor checks the preceding stored valuesof dZ/dt_(n) to look for a rapid rise in impedance. For example, a rapidrise in impedance may be detected using the following rule: for n=1 to10, dZ/dt_(n) must be greater than or equal to 1.0 for all 10. If thisrapid rise criteria is met at 320, the processor checks to see whetherthe required minimal time period has elapsed at 319, and if so,terminates ablation at 322 as discussed above. If the minimal timeinterval “b” has not elapsed, the microprocessor continues to acquiremeasurements of impedance and continues to calculate values of dZ/dt and|dZ/dt|_(n) until either an impedance plateau is detected at 318 or arapid rise in impedance is detected at 320 and the minimum time intervalhas expired at 319.

[0046]FIG. 7 is a function flow chart illustrating the over-alloperation of the device in conjunction with a multi electrode or multielectrode pair ablation apparatus. In the flow chart of FIG. 7, all theelectrodes are activated simultaneously and individual electrodes orelectrode pairs are deactivated in response to impedance measurementsassociated with the electrode pair indicating that the lesion formedbetween that electrode pair is completely transmural. In thiscircumstance, the ablation system works as follows.

[0047] After initialization at 406, all electrodes 1−x are activated at402, meaning that ablation energy is provided to all electrodes andelectrode pairs. The microprocessor then checks the value of dZ/dtand/or |dZ/dt| at the first of the electrode pairs at 404, using any ofthe mechanisms discussed above in conjunction with FIGS. 5a, 5 b and 6.At 408, the microprocessor checks using any of the mechanisms describedabove to determine whether an impedance plateau has been reached or, inthe event that additional criteria for shut off of ablation energy areprovided, to see whether any of these additional criteria are reached.If so, the electrode being examined is deactivated at 410 by ceasing thedelivery of ablation energy to the electrode or electrode pair. If not,the microprocessor checks the value of dZ/dt and/or |dZ/dt| for the nextelectrode at 406. This process continues until all electrodes aredeactivated at 412, after which the procedure is deemed complete at 414and the ablation process is terminated at 416.

[0048]FIG. 8 illustrates a functional flow chart of overall operation ofa device in which a multi-electrode or multi-electrode pair ablationapparatus is employed, as in FIG. 7. In this embodiment, however,electrodes or electrode pairs are activated initially and sequentially.After initialization at 500, the microprocessor activates delivery ofablation energy to the first electrode at 502, checks dZ/dt and/or|dZ/dt| at 504 and, in the same manner as described for FIG. 7 above,checks to determine whether the impedance plateau criteria and/or othercriteria as described above have been met. If so, the electrode isdeactivated at 510. If not, application of ablation energy continuesuntil the plateau criterion or other criteria are met as describedabove. After deactivation of an electrode at 510, the microprocessorchecks to determine whether all electrodes have been activated anddeactivated at 512, if not, the microprocessor then activates the nextelectrode at 506 and initiates delivery of ablation energy to thatelectrode. This process continues until the last electrode has beendeactivated at 512, following which the microprocessor determines thatthe ablation process is complete at 514 and the ablation process isstopped at 516.

[0049] The overall operational methodology of FIG. 7 is believed to bedesirable in that it allows for a more rapid completion of an ablationprocedure. However, the overall operational method is described in FIG.8 has the advantage that it may allow the use of a somewhat simplifiedgenerator because that multiple, separate impedance measurementcircuits, power control circuits, and the like need not be provided foreach electrode or electrode pair. A simple switching mechanism may beused in conjunction with only a single RF generator and impedancemeasurement circuit to successively apply energy to each electrode andto monitor impedance according to the invention.

[0050] FIGS. 9-12 show yet another aspect of the invention in which RFpower delivery can also be controlled in order to assist in theassessment of lesion transmurality. The ablation commences at a firstpower level and is then increased to a second power level as certainconditions are satisfied. The power level can also be increased from thesecond power level to a third power level and in the same stepwisemanner, increasing the power level from an N power level to an n+1 powerlevel until a set of conditions are met that indicates thattransmurality has been achieved and/or that the power should be turnedoff. For example, an initial power level P₀ can be selected in the rangeof about 10 to 25 watts. The power level can then be rapidly increasedto a second power level P₁ in an increment in the range of about 3 to 10watts at a rate of at least about 0.5 watts/second. Subsequent powerlevels P₂, P₃, etc. can be similarly achieved. A maximum permitted powerin the range of about 25-50 watts can also be selected to ensure thatthe power level is not raised beyond safe limits. Typical power levelscould be an initial power level P₀ of about 25 watts and a second powerlevel P₁ of about 30 watts. Power levels P2, P3, etc. could be increasedin similar increments until a maximum power level of about 40 watts wasachieved.

[0051] In each of the FIGS. 9-12, an initial selected power level P₀ iseither maintained or increased to a second power level in response tochanges in the detected impedance occurring during the ablation. In eachcase, the initial detected impedance Z₀ drops initially as ablationproceeds. However, each of FIGS. 9-12 shows exemplary alternativeresponses of a system according to the invention as differing impedanceconditions are detected. In FIG. 9, the power level remains the sameduring the entire ablation procedure because a detected rapid rise inimpedance dZ/dt_(rise) is detected without first detecting a plateau.One condition indicating that transmurality has been achieved is a rapidrise in impedance such as that shown in FIG. 9. Upon detecting a rapidrise in impedance, the power is turned off and the ablation for thelesion is completed. Conditions for an increase in power level caninclude one or more of: the detection of a plateau in impedance or aslow rise in impedance or the achievement of a maximum permitted timefor that power level. Conditions indicating that transmurality has beenachieved can include one of: the lack of change in detected impedance inresponse to the change in power level or the detection of a rapid risein impedance. FIG. 10 shows the detection of a plateau in the impedancecurve at dZ/dt₁, which triggers a rapid increase in power level from P₀to P₁. The increase in power level causes a drop in impedance andtherefore the ablation is continued at the second power level P₁ untilthe detected impedance indicates a rapid rise dZ/dt_(rise). Upondetection of the rapid impedance rise, the power is turned off and theablation for the lesion is completed. In the event that neither aplateau or a rapid rise is detected, ablation at the power level P₀ maybe continued for a maximum pre-selected period of time followed by anincrease in power level to power level P₁. For example, a time in therange of about 3 to 20 seconds, or more preferably about 8 seconds couldbe selected as the maximum duration for ablation at a particular powerlevel. FIG. 11 shows the detection of a plateau in impedance at dZ/dt₁which triggers an increase in power level from P₀ to P₁. Because theimpedance drops in response to the increased power level, ablationcontinues at the second power level P₁ until a second plateau inimpedance is detected at dZ/dt₂. The detection of the second impedanceplateau triggers a second increase in power level, from P₁ to P₂.Because the impedance drops in response to the increase in power level,ablation is continued at the third power level P₂ until a rapid rise inimpedance is detected at dZ/dt_(rise). Upon detection of the rapidimpedance rise, the power is turned off and the ablation for the lesionis completed. FIG. 12 shows the detection of a plateau in impedance atdZ/dt₁, which triggers an increase in power level from P₀ to P₁. Becausethe impedance drops in response to the increased power level, ablationcontinues at the second power level P₁ until a second plateau isdetected at dZ/dt₂. The detection of the second plateau triggers asecond increase in the power level from P₁ to P₂. Because the impedancedoes not drop in response to the increase in power level from P₁ to P₂,the continued plateau is verified by impedance measurement at dZ/dt₃ andthe ablation is immediately terminated. Each of FIGS. 9-12 illustratessuccessful termination of ablation by detecting impedance that ischaracteristic of achievement of transmurality. If the detection ofimpedance does not indicate the achievement of transmurality, theablation can still be terminated if the total time for the ablationexceeds a predetermined limit. For example, a total limit on ablation toform the lesion could take about 30 to 60 seconds, and preferably about40 seconds. Alternatively, ablation could also be stopped upon reachinga maximum power level or after the completion of a predetermined numberof power level increases.

In conjunction with the above specification, we claim:
 1. A method ofablation, comprising: applying ablation energy to a first tissue site;monitoring impedance of the first tissue site; and responsive tooccurrence of an impedance plateau at the first tissue site, increasingapplication of ablating energy to the first tissue site.
 2. A method asin claim 1 wherein the applying of ablation energy to the first tissuesite comprises applying RF energy using a first RF electrode.
 3. Amethod as in claim 2 wherein the monitoring of impedance of the firsttissue site comprises monitoring impedance using the first RF electrode.4. A method as in claim 2 wherein monitoring of impedance of the firsttissue site comprises monitoring voltage and current measurements toderive impedance.
 5. A method as in claim 1 wherein monitoring ofimpedance of the first tissue site comprises monitoring impedance usingdedicated monitoring electrodes.
 6. A method as in claim 1 wherein theapplying of RF energy to the first tissue site comprises applying RFenergy using an irrigated first RF electrode.
 7. A method as in any ofclaims 1-6, wherein the increasing of application of ablating energy tothe first tissue site comprises increasing application of ablationenergy responsive to a series of monitored impedances that have anacceptable degree of change.
 8. A method as in any of claims 1-6,wherein the increasing of application of ablating energy to the firsttissue site comprises increasing application of ablation energyresponsive to a series of monitored impedances that have an acceptablerate of change.
 9. A method as in claim 1, further comprising: applyingablation energy to a second tissue site; monitoring impedance of thesecond tissue site; and responsive to occurrence of an impedance plateauat the second tissue site, increasing application of ablating energy tothe second tissue site.
 10. A method as in claim 9 wherein the applyingof ablation energy to the second tissue site comprises applying RFenergy using a second RF electrode.
 11. A method as in claim 10 whereinthe monitoring of impedance of the second tissue site comprisesmonitoring impedance using the second RF electrode.
 12. A method as inclaim 10 wherein monitoring of impedance of the second tissue sitecomprises monitoring voltage and current measurements to deriveimpedance.
 13. A method as in claim 9 wherein monitoring of impedance ofthe second tissue site comprises monitoring impedance using dedicatedmonitoring electrodes.
 14. A method as in claim 9 wherein the applyingof RF energy to the second tissue site comprises applying RF energyusing an irrigated second RF electrode.
 15. A method as in any of claims9-14, wherein the increasing of application of ablating energy to thesecond tissue site comprises increasing application of ablation energyresponsive to a series of monitored impedances that have an acceptabledegree of change.
 16. A method as in any of claims 9-14, wherein theincreasing of application of ablating energy to the second tissue sitecomprises increasing application of ablation energy responsive to aseries of monitored impedances that have an acceptable rate of change.17. A method as in any of claims 9-14, wherein the applying of ablationenergy to the first and second ablating means is initiatedsimultaneously.
 18. A method as in any of claims 9-14, wherein theapplying of ablation energy to the second ablating means is initiatedfollowing termination of application of ablation energy to the firstablating means.
 19. A method of ablation, comprising: applying ablationenergy to a first tissue site; monitoring impedance of the first tissuesite; responsive to occurrence of an impedance plateau at the firsttissue site, increasing application of ablating energy to the firsttissue site; and responsive to occurrence of a rise in impedance at thefirst tissue site, modifying application of ablating energy to the firsttissue site.
 20. A method as in claim 19 wherein the applying ofablation energy to the first tissue site comprises applying RF energyusing a first RF electrode.
 21. A method as in claim 20 wherein themonitoring of impedance of the first tissue site comprises monitoringimpedance using the first RF electrode.
 22. A method as in claim 20wherein monitoring of impedance of the first tissue site comprisesmonitoring voltage and current measurements to derive impedance.
 23. Amethod as in claim 20 wherein monitoring of impedance of the firsttissue site comprises monitoring impedance using dedicated monitoringelectrodes.
 24. A method as in claim 20 wherein the applying of RFenergy to the first tissue site comprises applying RF energy using anirrigated first RF electrode.
 25. A method as in claim 19 wherein themodifying of application of ablating energy comprises termination ofablating energy.
 26. A method as in any of claims 19-25, wherein theincreasing of application of ablating energy to the first tissue sitecomprises increasing application of ablation energy responsive to aseries of monitored impedances that have an acceptable degree of change.27. A method as in any of claims 19-25, wherein the modifying ofapplication of ablating energy to the first tissue site comprisesmodifying application of ablation energy responsive to a series ofmonitored impedances that have an acceptable degree of change.
 28. Amethod as in any of claims 19-25, wherein the increasing of applicationof ablating energy to the first tissue site comprises increasingapplication of ablation energy responsive to a series of monitoredimpedances that have an acceptable rate of change.
 29. A method as inany of claims 19-25, wherein the modifying of application of ablatingenergy to the first tissue site comprises modifying application ofablation energy responsive to a series of monitored impedances that havean acceptable rate of change.
 30. A method as in claim 19, furthercomprising: applying ablation energy to a second tissue site; monitoringimpedance of the second tissue site; responsive to occurrence of animpedance plateau at the second tissue site, increasing application ofablating energy to the second tissue site; and responsive to occurrenceof an impedance rise at the second tissue site, modifying application ofablating energy to the second tissue site.
 31. A method as in claim 30wherein the applying of ablation energy to the second tissue sitecomprises applying RF energy using a second RF electrode.
 32. A methodas in claim 31 wherein the monitoring of impedance of the second tissuesite comprises monitoring impedance using the second RF electrode.
 33. Amethod as in claim 31 wherein monitoring of impedance of the secondtissue site comprises monitoring voltage and current measurements toderive impedance.
 34. A method as in claim 30 wherein monitoring ofimpedance of the second tissue site comprises monitoring impedance usingdedicated monitoring electrodes.
 35. A method as in claim 30 wherein themodifying of application of ablating energy comprises termination ofablating energy.
 36. A method as in claim 31 wherein the applying of RFenergy to the second tissue site comprises applying RF energy using anirrigated second RF electrode.
 37. A method as in any of claims 30-36,wherein the increasing of application of ablating energy to the secondtissue site comprises increasing application of ablation energyresponsive to a series of monitored impedances that have an acceptabledegree of change.
 38. A method as in any of claims 30-36, wherein themodifying of application of ablating energy to the second tissue sitecomprises modifying application of ablation energy responsive to aseries of monitored impedances that have an acceptable degree of change.39. A method as in any of claims 30-36, wherein the increasing ofapplication of ablating energy to the second tissue site comprisesincreasing application of ablation energy responsive to a series ofmonitored impedances that have an acceptable rate of change.
 40. Amethod as in any of claims 30-36, wherein the modifying of applicationof ablating energy to the second tissue site comprises modifyingapplication of ablation energy responsive to a series of monitoredimpedances that have an acceptable rate of change.
 41. A method as inany of claims 30-36, wherein the applying of ablation energy to thefirst and second ablating means is initiated simultaneously.
 42. Amethod as in any of claims 30-36, wherein the applying of ablationenergy to the second ablating means is initiated following terminationof application of ablation energy to the first ablating means.